Method of reversing in situ combustion frontal movement



March 9, 1965 J. c. TRANTHAM ETAL 3,172,467

METHOD OF REVERSING IN SITU COMBUSTION FRONTAL MOVEMENT Filed 001;. 8, 1962 2 Sheets-Sheet 1 'CORE HOLES OTEM PERATUR E OBSERVATION WELLS @INJECTION WELLS INVENTORS J.C. TRANTHAM A.R. SCHLEICHER A TTORNEYS United States Patent 3,172,467 METHOD OF REVERSENG EN SITU COMBUSTION FRONTAL MOVEMENT Joseph C. Trantham and Arthur Richard Schlelcher, Bartlesville, Okla., assignors to Phillips Petroleum Company, a corporation of Delaware Filed Oct. 8, 1962, Ser. No. 228,891 13 Claims. (Cl. 166-4) This invention relates to a process for the recovery or production of fluid hydrocarbons from underground formations containing hydrocarbon material, as applied to primary, secondary, or tertiary recovery programs. A specific aspect of the invention pertains to the recovery of hydrocarbon material by in situ combustion.

This application is a continuation-in-part of our application Serial No. 754,159, filed August 11, 1958 (now abandoned), which in turn is a continuation-in-part of our application Serial No. 529,916, filed August 22, 1955 (now U.S. Patent 3,126,955).

A process known as inverse air injection in an in situ combustion method of recovering hydrocarbons from an underground hydrocarbon-containing formation is disclosed in the copending application of John Marx, entitled Oil Recovery Process, Serial No. 526,388, filed August 4, 1955, now abandoned. In the disclosed meth od or process a fire front or combustion front is established in a formation surrounding a borehole in the formation by injection of hot combustion-supporting gas through the borehole into the formation (or by other suitable means) and after the combustion front has been established, the injection of air into the borehole is terminated and air or other combustion-supporting gas is injected into surrounding boreholes and forced to the combustion front or area so as to continue the combustion and advance the combustion front in a direction countercurrent to the flow of air to the burning area. If the injection of oxygen-containing gas is continued the combustion Zone or front moves through the formation to the injection point or borehole. In this manner as the combustion front traverses the formation between the injection borehole and the borehole at which combustion was initiated, the fluid hydrocarbons freed from the formation by the combustion and the passage of hot gas through the burned-out area back of the combustion front are driven into the borehole in which combustion was initiated and are there produced or recovered in gaseous and liquid form by conventional methods. In this inverse air injection technique, as applied to an oil-bearing underground formation, the burned-out or coked area back of the flame or combustion front retains a substantial proportion of the hydrocarbon material driven out of the combustion area as the combustion front advances, even though the reverse injection method in in situ combustion produces from about 20 to 35 or 40 percent or more of the hydrocarbon initially present in the formation. Of course, a substantial portion of the hydrocarbon initially in place in the formation is consumed as fuel in the combustion process.

We have found that continued injection of air or other oxygen-containing combustion-supporting gas into the projection well or wells after the flame or combustion has arrived at the injection well causes a reversal of the direction of the front and continued injection of the gas drives the front back to the area of initiation of the combustion and to the producing borehole, thereby substantially completely depleting the formation. We have also found that the inversely moving front can be caused to reverse itself at any stage of the process, before it reaches the injection well, and be driven back to its source. Various techniques can be utilized to effect the reversal. A reduction in velocity of injected 3,1?ZA67 ?aitented Mar. 9, 19%5 air to a value below about 28 s.c.f.h./ft. (standard cubic feet per hour per square foot of cross section of combustion front) results in reversal of the combustion front, as does the complete stoppage of air flow, for a period of about one hour or more. Likewise, injection of a slug of an inert gas such as CO N etc., for a similar period causes reversal. Reversal of air flow, whereby air is injected thru the production borehole for a period of at least one hour, also effects reversal of the front movement when injection thru the air injection borehole is resumed. Reducing the 0 concentration in the injected gas below about 8 or 10% for at least an hour also ellects reversal. While a value of 20 s.c.f.h./ft. is an average value, this rate will vary slightly according to the characteristics of the stratum.

We are not certain why the front reverses when the O supply is cut off or reduced for a short period and 0 (air) injection is then resumed. The best theory appears to be that the front ceases to advance and thereby fails to provide fuel at the leading edge of the front to cause the required advancement. Unless volatile fuel is available in the leading edge of the front, no burning takes place there and movement of the burning front in the hot unburned carbon residue in the direction of gas flow takes place. However, the invention is not to be limited by any theory regarding the mechanism of reversal of the front. It is sufficient that the steps to be performed to effect reversal of the front from a reverse (or inverse) to a direct burning front are taught herein.

The principal object of the invention is to provide a process or method of recovery which recovers a substantial proportion of the hydrocarbon material remaining in an underground formation after recovery of hydrocarbons therefrom by inverse injection in situ combustion. Another object of the invention is to provide an oil recovery process which substantially completely depletes an underground oil-bearing formation of hydrocarbon material when applied to the formation after inverse injection of combustion-supporting gas in in situ combustion techniques. A further object is to provide an in situ combustion oil recovery method which substantially upgrades the hydrocarbon material contained in the formation and effects recovery thereof. It is also an object of the invention to provide a second-stage concurrent flow combustion recovery method which supplements inverse injection in situ combustion recovery to substantially deplete the formation. Another object of the invention is to provide an in situ combustion process which permits wider spacing of boreholes and more rapid recovery of hydrocarbons from a large section of stratum. Other objects of the invention will become apparent from a consideration of the accompanying disclosure.

One aspect of the present invention comprises continuing the injection of combustion-supporting gas into the injection borehole at the normal termination of the burning step in an inverse air injection process wherein the combustion or fire front is moved through the formation from the borehole in which combustion is initiated to the injection borehole countercurrently to the flow of the combustion-supporting gas, so as to then reverse the direction of movement of the combustion front and drive the same back through the formation by direct drive toward the production borehole at which combustion was initiated. The process of reversing the direction of the combustion front is effected without reignition or supplying of heat other than that produced in the continuing combustion as the front reaches the injection borehole, i.e., the reversal is automatic with continuing injection of combustion-supporting gas. in this manner the coked material formed in the formation by the first stage combustion and/ or the hydrocarbon material trapped by the burned-out area from the ice 'borehole(s).

hydrocarbon-containing stream flowing to the production borehole from the combustion front are burned in part so as to substantially completely drive out all of the remaining hydrocarbon material from the formation and leave the formation substantially completely depleted of hydrocarbon material in the wake of the second state combustion front.

Another aspect of the invention comprises reversing the combustion or fire front from an inverse (or reverse) burning front to a direct burning when the front is intermediate the injection and ignition (production) wells and spaced substantially from both wells. This reversal at any stage of the inverse burning phase of the process is effected broadly by reducing the rate of supply to the combustion front below a level required to maintain the inverse movement thereof as set forth herein above.

Hence, the inverse burning front can be reversed to a direct burning front at any stage from the time it moves into the stratum from the ignition well until it reaches the injection well. It automatically reverses when it runs out of forward fuel at the injection well and when forward fuel is cut off purposely at any intermediate stage of the inverse burning phase.

In accordance with the process of the invention, combustion is initiated by any suitable means in a borehole in the hydrocarbon-bearing formation from which hydrocarbons are to be produced and after a combustion area surrounding the borehole has been established, injection of combustion-supporting as such as air from one or more surrounding boreholes is commenced so as to force the gas through the formation to the combustion area and cause the combustion or fire front to move through the formation toward or to the injection When the combustion front is allowed to reach the injection borehole(s), the flow of oxygen-containing gas is continued and the combustion front is driven in a reverse direction (by direct drive) to the borehole in which combustion was initiated, thereby pro ducing fluid hydrocarbons from the formation during both the first (inverse) and second (direct) stages or phases of combustion and the hydrocarbon material is recovered from the borehole in which combustion was initiated. The produced hydrocarbons include the condensible hydrocarbon vapors produced by thermal decomposition or cracking in situ, as well as fluid hydrocarbon material rendered fluid by heat obtained from the combustion process. hydrocarbons are in vapor form due to the high temperature created in the combustion zone and the passage of the produced fluids thru the hot stratum behind the inverse burning front.

It has been found that the in situ combustion process described herein raises the gravity of the original hydrocarbon in the formation from about API to the range of 20 to 30 API depending upon the combustion temperature maintained in the formation during the movement of the combustion front therethrough and other factors, such as the nature of the formation.

In one particular instance in field operation, an oil of 10 AF! was increased to 23 API at a combustion temperature of approximately 1000 F. and the viscosity of the produced hydrocarbon was approximately only half that expected of an oil of this gravity and only a small fraction of that of the original oil. It has been found that combustion can be adequately supported at temperatures in the combustion zone of about 750 to 800 F. on the first stage combustion and this temperature usually rises about 200 to 300 F. with substantially the same rate of air injection in the second stage combustion in which the flow of gas and combustion front are concurrent. Temperatures in the range of 1400 to 1600 F. have been maintained in the combustion front during the second stage combustion but the amount of cracking of the hydrocarbon material probably is sub- Usually, all of the produced,

stantially greater during inverse injection of air than that effected in the second stage combustion at significantly higher temperatures, due to driving the hydrocarbons ahead of the combustion front and not through I it, as in first stage combustion.

Preferred operating temperatures in the first stage combustion (inverse air injection) are in the range of 750 to 1000 F., and in the second stage (concurrent air flow and movement of combustion Zone) are in the range of 1000 to 1800 F.

In order to illustrate the invention, reference is made to a test made on a representative tar sand of approximately 8.0 weight percent tar saturation. The tar sand in particulate form was packed into a 1% 1.1)., 304 stainless steel tube about 37" long by tamping the sand as it was placed in the tube. The tube was heat insulated to simulate underground conditions. Seven thermocouples spaced along the length of the tube projected into the sand at intervals of 2 and 4" between the first and second and between the second and third thermocouples, respectively, and there were 6" intervals between the third and fourth thermocouples and between each pair of succeeding thermocouples to the opposite end of the tube. A cap on each end of the tube was provided with connections and conduits for introducing and withdrawing gas from the tube so that air injection could be effected at either end of the tube and withdrawal of fluid effluent from the'other connection could be simultaneously made from the opposite end. An electric heater was installed at a position close to the number 1 thermocouple and this electric heater comprised a porous disc positioned across the end of the tube so that gas passed through the heater before entering the sand. The current supplying the heater was turned on so as to warm up the heater and nitrogen was introduced to'the heater-end of the tube and when the temperature at the first thermocouple reached 600 F. the flow of nitrogen was cut off and air was passed through the heater and into the sand. The temperature at the first thermocouple rose immediately, indicating that ignition or combustion of the tar had begun. The introduction of air was continued until the temperature rose suddenly at the second thermocouple, indicating that the combustion or fire front had reached this thermocouple which was'approximately 2" from the end of the column of sand. At this point the injection of air was reversed so that the air traveled from the opposite end of the tube to the combustion front. The combustion continued with inverse air injection as indicated by the progression or advance of the combustion front to the third thermocouple. In due course the temperature rose in succession at thermocouples 4, 5, 6, and 7, indicating that the combustion front had moved completely through the column of sand. Air injection was continued and the temperature of the seventh and last thermocouple, adjacent the incoming stream of air, rather suddenly rose to approximately 1400 F. and then slowly dropped. Within a short time the temperature at thermocouples 6, 5, 4, 3, 2, and 1 rose in succession in that order to a temperature in the neighborhood of 1400 F., indicating that the combustion front had progressed back through the tube concurrently with the flow of air and gas therein; The effluent fluid was recovered from the end of the tube adjacent the No. 1 thermocouple for purposes of analyses.

During inverse injection of air (first stage combustion) the average flow rate of air was maintained in the range of 2000 to 2250 cc./minute and the average space velocity was 200 to 250 s.c.f.h./ft. (standard cubic feet per hour per square foot). The temperatures recorded at the various thermocouples were in the range of 1000 to 1050 F.

During the second passage of the combustion front through the tube (second stage combustion) the air flow rate was maintained in the range of 2500 to 2900 cc./minnte, amounting to a space velocity in the range of 270 to 320 s.c.f.h./ft. The temperatures during this stage as recorded at the various thermocouples were in the range of 1400 to 1470 F. Other runs were made with comparable results, indicating that a temperature in the range of 750 to 1009 F. may be readily maintained during the first stage combustion while a temperature in the range of 1000 to 1800 F. may be readily maintained during the second stage combustion.

The gravity of the original tar was found to be about API and viscosity was considerably greater than 100 ccntipoises. The hydrocarbon portion of the effluent obtained during inverse air injection (first stage combustion or first sweep) was found to have gravity of 22.1 API and a viscosity of 18.6 centipoises. The hydrocarbon elfluent from the second stage combustion or second sweep had a gravity of 20.0 API and a viscosity of 53.7 centipoises. (API gravity at 60 F. and viscosity at 100 F.) These results are comparable to those obtatined in other runs and are fairly typical.

Temperature control of the combustion zone may be effected by regulation of the flow rate of co-mbustion-sup porting gas and/ or by varying the oxygen concentration therein. One effective method comprises admixing with air some of the combustion gas recovered from the production borehole or from other available sources. Higher concentrations of oxygen (than in air) may be provided by conventional means.

It was surprising and unexpected to find that the reversal of the direction of travel of the fire or combustion front took place when the burning zone reached the end of the tube or packed column of tar-containing sand. Examination of sand taken from the packed tube, following an inverse air injection run and before the combustion front was passed through the tube in the opposite direction, revealed that considerable coke and trapped hydrocarbons are present in the sand. Examination of the sand taken from a "packed tube after traversal thereof by the combustion front in both directions shows that the hydrocarbon material is substantially completely removed and the sand has the color of the formation with the hydrocarbon completely removed therefrom. In other words, the sand has substantially the same color as when the tar sand is calcined in open in contact with air until all of the hydrocarbon material is removed and the sand is brought to its normal color. This indicates that the process of the invention is adapted to substantially completely deplete an underground formation at least in the accessible burning area thereof.

In order to further illustrate the invention, reference is made to the drawing and to field tests to which the drawing relates. FIGURE 1 is a plane view of a well pattern utilized in field tests; FIGURE 2 is a graph showing the time-temperature history of one of the Wells in the plot of FIGURE 1; and FlGURE 3 is a graph or curve showing the relationship between fire front velocity and formation air velocity in field tests.

A number of field tests were conducted in tar sands in the vicinity of Bellamy, Missouri, utilizing inverse air injection wherein the combustion zone was caused to move thru the tar sand countercurrently to the flow of combustion air.

FIGURE 1 illustrates the well pattern utilized in one of the tests. In this pattern a central well 100 is surrounded by a ring of nine equally spaced injection Wells designated 101, 102, 103, 104, 165, rue, 107, 108 and 109. Each injection well is on a 25 ft. radius from the central well. Thermocouple or temperature observation wells were drilled at regular intervals along the radius between the center well 100 and each of the nine wells in the ring, there being three thermocouple wells on each radius with thermocouples positioned in each well at four different levels within the 6 ft. thick tar sand. Of these thermocouple wells only wells 123, 124, and 141 are shown in the drawing along with core holes 119a, 1190, and

6 119d. Well 141 and core hole 119d are on 5 ft. radii, well 123 is on a 9 ft. radius, core hole 119a is on a 10 ft. radius, and well 124 and core hole 1190 are on 11 ft. radii.

After drying out the tar sand in the 50 foot circle covered by the well pattern by injecting air thru the sand between the center well and the wells in the ring, ignition of the tar sand was effected at the center well and the resulting combustion front was propagated toward the ring wells by injecting air thru the ring wells and producing thru the center well. During the fire front propagation the injection pressure was varied and the air velocities also varied from about 45 standard cubic feet per square foot of cross section of sand per hour (s.c.f.h./ft. to substantially below 20 's.c.f.h./ft.

The movement of the fire front was traced in three directions along the line of the thermocouple wells, a rise in temperature at a thermocouple indicating the arrival of the fire front and this was followed by a drop in temperature at this particular thermocouple. When the air velocity was reduced below about 20 s.c.f.h./ft. after the fire front passed a thermocouple and the temperature rose and then receded, it was noted that the temperature at this thermocouple again rose and receded as if a fire front were again passing this thermocouple. After continuing the low rate of air injection or air velocity for an extended time, cores were taken between the thermocouple wells and the ignition or production well (center well) and it was found that the sand was burned clean, it being almost white in color. Core samples were taken at several locations between a thermocouple well and the central well after it appeared that the flame front had passed the thermocouple well moving outwardly toward the wells in the ring and then had moved back past the thermocouple well toward the central well, upon reduction of the air velocities below about 20 s.c.f.h./-ft.

The temperature record for well 124 shows that the temperature in this well, at a radius of 11 ft., never reached the ignition temperature (550 F.), the highest recorded temperature being 400' F. Core analyses in well 1190 (same radius) showed small Zones of coked sand, but no clean-burned sand. Our laboratory experiments have shown that invariably a coked zone precedes the fire front in counterflow (inverse) combustion because of the heat conducted ahead of the fire front and sweeping of this sand by the air stream.

The temperature in well 123 rose to levels as high as 1095 F. but did not reach this value till near the end of the test and did not decline during air injection. Thus, the combustion zone never progressed far enough past this well to permit the temperature to decline. Core analoccurred bet-ween wells 1 23 and 124. Calculation of the air velocity at the combustion zone, using the gas flow from the production well during this time and the radius of the combustion front based on the temperature readings gives a value of -19.6 s.c.f.h./ft. Thus, since the combustion front did not reach well 124 but burned past well 123 and then "back, the air flux causing this was quite close to 19 s.c.f.h./ft. or just below 20 s.c.f.h./ft.

As evidence that this was not merely a temporary burnback, the temperature record in well 141 is plotted in FIGURE 2. It is seen that there are two temperature maxirna corresponding to the passage of the counterfiow front and later the rate of the burnback or passage of the direct burning front past well 141. The core analysis of core hole 119d at the same radius as well 141 showed clean-burned sand to confirm the conclusions reached.

It is well known in the in situ combustion art that an inversely moving combustion front leaves a black carbonaceous residue in the sand behind the combustion front. It is also known that a direct drive combustion front burns the sand substantially clean. This has been demonstrated in the laboratory on numerous occasions.

d It is therefore obvious from the foregoing data that the combustion front moved out past a thermocouple well by inverse drive and reversed and moved back past the same thermocouple well by direct drive upon reduction of the air velocity below about 20 s.c.f.h./ft.

The graph of FIGURE 2 shows the temperature curve of Well 141 in the aforesaid test pattern. The first peak temperature is sharp and is typical of a reverse burning front while the second peak temperature is much more gradual and is typical of a direct burning front. About 165 hours after ignition, the air flux was reduced below about 20 s.c.f.h./ft.' which apparently caused the combustion or fire front to reverse and pass back thru the sand adjacent the thermocouple well 1141 thereby gradually producing another peak temperature.

The curve or graph of FIGURE 3 was derived from data obtained in a line-drive test made between two parallel lines of spaced apart wells and from the fieldtest described in connection with FIGURES l and 2. The relationship between fire front propagation velocity (Vf) and formation air velocity (S), as determined from field measurements on the -foot-thick Bellamy pay zone, can

be described by the empirical equation:

V =0.013 l (S--S Here V =fire front propagation rate, in feet/ hour; S :forrnation air velocity, in s.c.f.h./ft. and S =limiting formation air velocity, taken as 19 s.c.f.h./ft.

The curve clearly demonstrates that the fire front has zero outward velocity when the formation air velocity is slightly less than 20 s.c.f.h./ft. This collaborates with and corroborates the evidence that the fire front actually reverses and moves back to the ignition well and low formation air velocity below about 20 s.c.f.h./ft.

Experience in the Bellamy field tests clearly demonstrates that a counterflow or reverse burning front in an in situ combustion process is caused to reverse to a direct burning front by reducing the air flux below about 20 s.c.f.h./ft. This reversal may also be effected by injection of a slug of non-oxidizing gas, or by termination of injection for a short period. The effect may be produced also by reducing the 0 content of the injected combustionsupporting gas, such as below about at rates of injection above s.c.f.h./ft. In the Bellamy tests, reversal of the inverse burning front was effected by both lowering the air fiux to less than 20 s.c.f.h./ft. and by completing compressor shut-down followed by resumption of air injection at combustion supporting rates such as about 20 s.c.f.h./ft. The time required for cooling the leading edge of the combustion or fire front sufiicient to deprive the leading edge of the necessary volatile hydrocarbon material to continue the reverse burning for as long as about one hour. Longer periods may be utilized in effecting the reversal, the only requirement being that the temperature in the combustion area or hot zone created by the burning process not be allowed to cool below combustion-supporting temperatures.

We have discovered another technique of operation utilizing inverse and direct air injection which comprises initiating combustion around a plurality of in-line ignition boreholes and driving the combustion fronts by inverse drive toward two lines of air injection boreholes, one on each side of the line of ignition boreholes and generally parallel therewith, until the combustion fronts are substantially midway between the line of ignition boreholes and the lines of injection boreholes. At this point the movement of the fronts is reversed by any of the previously described methods so that they are driven back to the line of ignition boreholes. During both phases of the process, produced hydrocarbons are recovered thru the ignition boreholes, which may be considered production boreholes.

The next step in the process comprises igniting the stratum around the injection boreholes in each line and injecting air thru the production boreholes of the preceding operation so as to drive the combustion fronts toward the production boreholes by inverse drive. When the fronts arrive at the previously burned out areas, they automatically reverse. In this manner an inverse and a direct burning phase are passed only about half the distance betweenthe lines of boreholes which makes it feasible to utilize greater spacing between lines of boreholes and increases rate of production from a field.

In establishing ignition and a combustion front around a line of ignition boreholes, a preferred manner of operating comprises igniting the stratum around alternate boreholes in the line and utilizing the other boreholes in the line as air injection boreholes so as to drive the combustion front from each ignition borehole to the adjacent injection boreholes. Direct drive of the combustion front to the adjacent boreholes may also be practiced in some types of strata which are not subject to plugging by heavy liquid hydrocarbons. Of course, after the combustion front has been established alongthe line of ignition boreholes inthis manner, injection of air thru the parallel lines of injection boreholes is initiated and the process is effected as aforesaid,

It is also feasible to effect inverse drive of a combustion front between a first ring of boreholes immediately surrounding a central borehole and said central borehole and simultaneously between said first ring of boreholes and a second ring of outer boreholes generally concentric with said first ring, in similar manner to the operation described in connection with inline boreholes. Ignition is effected around said first ring and air is injected thru the central borehole and also thru the outer ring of boreholes so as to effect inverse drive of the combustion fronts toward the outer ring and toward the central borehole. When the combustion fronts have reached a selected intermediate area, usually midway, reversal is effected by reducing the flow of air below the rate required to maintain the inversely moving front so that the slower flow initiates driving the combustion front back thru the partially burned area. After reversal, the flow rate may of course be in creased in order to provide faster production rates. After the combustion fronts are moved partially thru the areas from the first ring of boreholes toward the outer ring and toward the central borehole by inverse drive and back by direct drive, the stratum is ignited around the central borehole and also around the outer ring of boreholes and air is injected thru the first ring of boreholes so as to move the combustion fronts to the burned-out area and back to the starting points. Simultaneously with the ignition of the stratum around the outer ring and with injection of air thru the first ring, air may be injected thru a third ring of concentric boreholes more remote from the central borehole so that the process is repeated and. production is extended radially outwardly from the central borehole as far as desired.

The process of the invention is applicable to the recovery of oil from formations which are amenable to recovery by inverse air injection in the in situ combustion technique. Hence, the process is applicable to primary, secondary, or tertiary recovery programs and is particularly applicable to the recovery of crudes too viscous to produce by other methods. A specific application is in the recovery of hydrocarbons deposited in shales and tar sands which present practically insurmountable difficulties when utilizing conventional recovery methods.

The original tar of the sand tested had a specific gravity of about 10 API and this was upgraded during first stage combustion recovery to approximately 23 A'PI gravity and the recovered oil had a viscosity of 20 cp. at F. or 106 secs. Saybolt.

The permeability of a formation may be increased, prior to application of the process thereto, by conventional means such as hydrofracing or sandfracing.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative details disclosed are not to be construed as imposing unnecessary limitations on the invention.

We claim:

1. A process for the underground combustion of a gas pervious carbonaceous deposit penetrated at spaced points by an injection well and a production well which comprises initiating a zone of combustion therein at a point adjoining said production well by heating and contacting the deposit at said point with an -containing gas; thereafter supplying O -containing gas (at less than combustion supporting temperature at the area of injection) thru said injection well to said zone to maintain said zone and to propagate same into said deposit away from said production well countercurrently to the flow of said gas; after said zone has been moved substantially away from said production well, and, while it is spaced substantially from said injection well, reversing the direction of movement thereof by reducing the O flow rate to said zone be low a sustaining rate for an inverse burning zone, and driving same back toward said production well by injecting O -containing gas to said zone; and recovering fluids resulting therefrom thru said production well.

2. The process of claim 1 wherein said zone is driven back to said production well.

3. The process of claim 1 wherein reversal of the counterfiow movement of said zone is efiected by reducing the O flow rate below that which occurs when air is injected at about 20 standard cubic feet per hour per square foot of cross section of combustion zone for a period of at least one hour and resuming O injection before the temperature of said zone falls below combustion-supporting temperatures.

4. The process of claim 3 wherein injection of said gas is terminated to stop the counterflow movement of said zone.

5. The process of claim 3 wherein said gas is air and the rate of flow is reduced substantially below 20 standard cubic feet per hour per square foot of cross section of combustion zone perpendicular to the direction of gas flow.

6. The process of claim 1 wherein the combustion temperature during countercurrent flow of gas and combustion front is in the range of about 750 to 1000 F. and during concurrent flow is in the range of about 1000 to 1800 F.

7. The process of claim 1 wherein a plurality of injection wells surrounding a central production well are used to move the combustion zone to the area of each injection well and back to said production well.

8. The process of claim 1 wherein a plurality of in-line injection wells and a plurality of in-line production Wells parallel therewith are used to move the combustion zone thru the deposit between said lines of wells toward the injection wells and back toward the production wells.

9. The process of claim 8 wherein said zone is moved counterflow to an area about midway between the lines of injection and production wells and then back to the line of production wells by direct drive followed by cutting ofr gas injection thru the injection wells and igniting said deposit around said injection wells, injecting said gas thru said production wells to the ignited areas around the injection wells so as to move the resulting combustion zones counterfiow to the flow of gas until same arrive at the burned out area.

10. The process of claim 9 including the step of continuing injection of said gas after the combustion zones arrive at said burned out area so as to drive same by direct drive back to said injection wells.

11. In a process for the underground combustion of a carbonaceous deposit, said deposit being penetrated at spaced points by an injection well and a producing well, the improvement which comprises injecting into said deposit an oxygen-containing gas, initiating a zone of combustion therein at a point adjoining said producing Well, thereafter supplying oxygen-containing gas through said injection well to said zone to maintain said zone and to propagate it through said deposit toward said injection well until said zone has reached an area a substantial distance from both of the wells, stopping the movement of said zone at said area by reducing the flow rate of said gas thereto below the sustaining rate for inverse movement thereof, subsequently further introducing oxygen-containing gas into said deposit through said injection well whereby the course or" said zone is reversed and travels concurrently with said gas toward said producing well, and recovering fluids resulting therefrom through said producing well.

12. A process for recovering hydrocarbons from a gas pervious carbonaceous stratum penetrated at spaced points by an injection well and a production well which comprises initiating a zone of combustion therein at a point adjoining said production well by heating and contacting the deposit at said point with an O -containing gas; thereafter supplying air (at less than combustion-supporting temperature at the area of injection) thru said injection well to said zone at a rate substantially above 20 s.c.i"'.h./ it? (standard cubic feet per hour per square foot of cross section of the combustion zone) so as to maintain said zone and propagate same counterflow to air into said stratum toward said injection well; when said zone has reached an area intermediate said Wells and spaced substantially therefrom, reducing the flow rate of air substantially below 20 s.c.f.h./ft. for at least one hour so as to stop the movement of said zone toward said injection well; thereafter continuing the flow of air at a combustion-supporting rate so as to move said zone by direct drive back toward said production well; and recovering produced fluids thru said production well.

13. The process of claim 12 wherein the counterflow movement of said zone is stopped by injecting a slug of non-oxidizing gas.

References fired in the file of this patent UNITED STATES PATENTS 2,841,375 Salomonsson July 1, 1958 2,888,987 Parker June 2, 1959 3,097,690 Terwilligcr et al. July 16, 1963 

1. A PROCESS FOR THE UNDERGROUND COMBUSTION OF A GAS PREVIOUS CARBONACEOUS DEPOSIT PENETRATED AT SPACED POINTS BY AN INJECTION WELL AND A PRODUCTION WELL WHICH COMPRISES INITIATING A ZONE OF COMBUSTION THEREIN AT A POINT ADJOINING SAID PRODUCTION WELL BY HEATING AND CONTACTING THE DEPOSIT AT SAID POINT WITH AN O2-CONTAINING GAS; THEREAFTER SUPPLYING O2-CONTAINING GAS (AT LESS THAN COMBUSTION SUPPORTING TEMPERATURE AT THE AREA OF INJECTION) THRU SAID INJECTION WELL TO SAID ZONE TO MAINTAIN SAID ZONE AND TO PROPAGATE SAME INTO SAID DEPOSIT AWAY FROM SAID PRODUCTION WELL COUNTERCURRENTLY TO THE FLOW OF SAID GAS; AFTER SAID ZONE HAS BEEN MOVED SUBSTANTIALLY AWAY FROM SAID PRODUCTION WELL, AND, WHILE IT IS SPACED SUBSTANTIALLY FROM SAID INJECTION WELL, REVERSING THE DIRECTION OF MOVEMENT THEREOF BY REDUCING THE O2 FLOW RATE TO SAID ZONE BELOW A SUSTAINING RATE FOR AN INVERSE BURNING ZONE, AND DRIVING SAME BACK TOWARD SAID PRODUCTION WELL BY INJECTING O2-CONTAINING GAS TO SAID ZONE; AND RECOVERING FLUIDS RESULTING THEREFROM THRU SAID PRODUCTION WELL. 